Hubble confirms new dark spot on Neptune

Space Telescope Science Institute Press Release

Neptune with dark spot (seen in blue light). Image credit: NASA, ESA, and M.H. Wong and J. Tollefson (UC Berkeley).New images obtained on 16 May 2016, by the NASA/ESA Hubble Space Telescope (HST) confirm the presence of a dark vortex in the atmosphere of Neptune. Though similar features were seen during the Voyager 2 flyby of Neptune in 1989 and by the HST in 1994, this vortex is the first one observed on Neptune in the 21st century.

The discovery was announced on 17 May 2016, in a Central Bureau for Astronomical Telegrams (CBAT) electronic telegram by University of California at Berkeley research astronomer Mike Wong, who led the team that analysed the Hubble data.

Neptune’s dark vortices are high-pressure systems and are usually accompanied by bright “companion clouds,” which are also now visible on the distant planet. The bright clouds form when the flow of ambient air is perturbed and diverted upward over the dark vortex, causing gases to likely freeze into methane ice crystals. “Dark vortices coast through the atmosphere like huge, lens-shaped gaseous mountains,” Wong said. “And the companion clouds are similar to so-called orographic clouds that appear as pancake-shaped features lingering over mountains on Earth.”The full visible-light image at left shows that the dark feature resides near and below a patch of bright clouds in the planet’s southern hemisphere. The dark spot measures roughly 3,000 miles (4,800 kilometres) across. Other high-altitude clouds can be seen at the planet’s equatorial region and polar regions.The full-colour image at top right is a close-up of the complex feature. Pancake-shaped clouds above the spot form when ambient air is perturbed and diverted upward over the vortex. The vortex is a high-pressure system.The image at bottom right shows that the vortex is best seen at blue wavelengths. Only Hubble has the high resolution required for identifying such weather features on distant Neptune. Image credit: NASA, ESA, and Z. Levay (STScI). Acknowledgment: M.H. Wong and J. Tollefson (UC Berkeley).Beginning in July 2015, bright clouds were again seen on Neptune by several observers, from amateurs to astronomers at the W. M. Keck Observatory in Hawaii. Astronomers suspected that these clouds might be bright companion clouds following an unseen dark vortex. Neptune’s dark vortices are typically only seen at blue wavelengths, and only Hubble has the high resolution required for seeing them on distant Neptune.

In September 2015, the Outer Planet Atmospheres Legacy (OPAL) program, a long-term Hubble Space Telescope project that annually captures global maps of the outer planets, revealed a dark spot close to the location of the bright clouds, which had been tracked from the ground. By viewing the vortex a second time, the new Hubble images confirm that OPAL really detected a long-lived feature. The new data enabled the team to create a higher-quality map of the vortex and its surroundings. Neptune’s dark vortices have exhibited surprising diversity over the years, in terms of size, shape, and stability (they meander in latitude, and sometimes speed up or slow down). They also come and go on much shorter timescales compared to similar anticyclones seen on Jupiter; large storms on Jupiter evolve over decades.

Planetary astronomers hope to better understand how dark vortices originate, what controls their drifts and oscillations, how they interact with the environment, and how they eventually dissipate, according to UC Berkeley doctoral student Joshua Tollefson, who was recently awarded a prestigious NASA Earth and Space Science Fellowship to study Neptune’s atmosphere. Measuring the evolution of the new dark vortex will extend knowledge of both the dark vortices themselves, as well as the structure and dynamics of the surrounding atmosphere.

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